Training load or intensity is often regarded as the most important variable to consider in the prescription of resistance exercise. For instance, loading schemes designed to increase strength through muscle growth or hypertrophy are typically characterized by loads of approximately 75% of one-repetition maximum (1RM), whereas schemes designed to improve strength (maximal) through neural coordination are typified by intensities of 85-100% 1RM (6,15,20). However, research suggests that such a perspective is somewhat simplistic because lighter loads (< 45% 1RM) have been found to be equally effective in promoting hypertrophy and/or inducing maximal strength gains compared with heavy-load training (10,12,19,23). Conversely, high-velocity movements using much lighter loads (e.g., 45% 1RM) are often thought to maximize speed and power development (27). Despite this approach to exercise prescription, many studies have found heavy-load training (> 70% 1RM) equally as effective as lighter loads in enhancing various measures of muscular power (1,11,21). Such findings illustrate an apparent lack of understanding of the stimuli afforded by different loading strategies and the adaptive response of the neuromuscular system to such stimuli.
The mechanical responses to resistance exercise (e.g., force, power, time under tension) would seem the most important stimuli for muscular adaptation (6). Most of our understanding about the kinematics and kinetics of resistance exercise is based on single repetitions and the effect of load intensity, as determined by the force- and power-velocity relationships (2,3,8,24-26). Heavy loads, for instance, are associated with high force production and greater time under tension, thereby providing the optimal stimulus for hypertrophy and maximal strength development. Conversely, lighter loads (e.g., 30-60% 1RM) optimize the mechanical power output of muscle, potentially leading to greater changes in power development and functional performance. These findings do not, however, reflect the true nature of the weight training (i.e., multiple repetitions, sets, and exercises) and the fact that workouts are characterized by different lifting techniques and varying rest periods. Hence, prescribing exercise based on single-repetition analysis would seem problematic. In conjunction with load, the total number of repetitions performed (i.e., volume) and the manner in which a given load is moved (i.e., technique) appear the major determinants of set kinematics and kinetics (4,5,7). Extrapolating these findings, the load effect is likely to be less important when attempting to elucidate the stimuli afforded by actual workouts, thereby providing a framework for understanding inconsistencies within research. Unfortunately, our understanding of the mechanical responses to resistance exercise is largely limited to single-repetition analysis.
A mechanical analysis of different loading schemes, especially those employed within practice, would provide a better understanding of the training stimulus and the expected adaptation occurring with the repeated application of such stimuli. Therefore, the aim of this study was to examine the mechanical responses to a power, hypertrophy, and maximal strength scheme. To emphasize the contribution of the different workout variables (volume, technique, and load), single-repetition and total-repetition responses were compared. On the basis of previous research, we hypothesized that repetition kinematics and kinetics would reflect the force- and power-velocity relationships and the load effect; however, volume and technique would play a greater role in determining the total-repetition responses that describe the “workout” stimulus.
Experimental Approach to the Problem
The mechanical responses to resistance exercise play an important role in mediating long-term adaptation. However, our understanding of these responses is largely based on single-repetition analysis, which does not reflect weight training practice (i.e., multiple repetitions, sets, and exercises). A mechanical analysis of different workouts employed within practice would provide a better understanding of the training stimulus. Thus, the present study examined the mechanical responses to a power, hypertrophy, and maximal strength scheme. Each scheme was performed using two squat exercises (supine and Smith) with the mechanical responses determined using a linear position transducer. To emphasize the contribution of the different workout variables (volume, technique, and load), single-repetition and total-repetition responses were compared.
Eleven males volunteered to participate in this study. The mean (± SD) age, height, and mass of the participants were 26.6 (6.7) years, 179.6 (6.2) cm, and 79.0 (8.1) kg, respectively. All subjects were healthy and characterized as experienced weight lifters, training at least 2 days per week (> 2 years), but none was considered a competitive lifter. The 1RM strength for the supine squat and Smith squat exercises were 226 (18) and 137 (13) kg, respectively. None of the subjects were taking any dietary or performance supplements. Each subject had the risks of the investigation explained to him and signed an informed consent form before participating in this study. The human subject ethics committee of the Auckland University of Technology approved all procedures undertaken.
Subjects performed their leg assessments and workouts on an isoinertial supine squat machine and a modified Smith machine (Fitness Works, New Zealand). The supine squat machine incorporated a 300-kg, pin-loaded weight stack attached to a sled on low-friction sliders, allowing horizontal squatting movements to be performed. The Smith machine, a common weight training device, allowed squatting movements to be performed in the vertical direction. With the use of additional free plates, the load for each exercise was adjustable in 2.5-kg increments. A mechanical brake on each machine was adjusted to ensure that subject knee angle (90°) at the bottom position was standardized between subjects and replicated across exercises. A linear transducer (P-80A; Unimeasure, Corvallis, Oregon-mean sensitivity 0.499 mV·V−1·mm−1, linearity 0.05% full scale) was attached to each apparatus and measured vertical load displacement with an accuracy of 0.01 cm. Validation of the linear transducer with a force platform (AMTI Force Plate and Amplifier: Advanced Medical Technology Inc., Watertown, Massachusetts) has been shown previously (9).
Subjects visited the test facility on four separate occasions. In the first session, subject 1RM was determined for the supine squat and Smith squat, using a repetition-to-failure protocol (13). During the next three sessions, subjects randomly completed a power (8 sets of 6 repetitions at 45% 1RM, 3-minute rest periods), hypertrophy (10 sets of 10 repetitions at 75% 1RM, 2-minute rest periods), and maximal strength scheme (6 sets of 4 repetitions at 88% 1RM, 4-minute rest periods) based on prescriptive guidelines described elsewhere (15,18). The power and maximal strength schemes were equated by load volume (i.e., total repetitions × load), with each workout approximately 22 minutes in duration. For all schemes, the supine squat preceded the Smith squat, with half the sets performed on each machine. Before assessment, subjects performed a warm-up consisting of two submaximal sets on the supine squat (5-10 repetitions, 75-125% body mass) and appropriate stretches. The maximal strength scheme was performed with a traditional technique (i.e., nonprojection), although subjects attempted to move the load explosively. In the power scheme, a ballistic technique (i.e., jump squats) was used on the supine squat, with the Smith squat performed in a similar manner, but only up to the toes to minimize the risk of injury, because no braking device was fitted to attenuate landing forces (25). Controlled lifting movements (1.5 seconds up, 1.5 seconds down) were performed in the hypertrophy scheme. The same investigator supervised all workouts and provided verbal encouragement throughout. To minimize the diurnal variation effect, each workout was conducted between 1400 and 1700 hours. A 2- to 3-day recovery period separated each session, with subjects instructed to replicate diet and hydration 1 day before their assessment.
Data were sampled at 200 Hz via a custom-built data-acquisition and analysis program (Labview 6.0). The filtered values were differentiated to determine velocity and acceleration data and were then used to calculate the mechanical variables of interest: force = mass (load in kilograms) × acceleration; contraction time; impulse = force × time; work = force × distance; and power = force × velocity (6). These variables were calculated for the eccentric and concentric phases of each squatting repetition, as well as for each set and exercise. The eccentric phase was defined as the period of time from maximum to minimum vertical displacement (i.e., lowering the load). The concentric phase was defined as the period of time from the minimum vertical displacement to maximum vertical displacement (i.e., lifting the load). Repetition data allowed direct comparison with other studies examining resistance exercise kinematics and kinetics using single repetitions, whereas total-repetition data gave an indication of those mechanical responses that describe the workout stimulus.
Standard statistical methods were used for calculating means (± SD) for the mechanical variables. The single-repetition and total-repetition mechanical responses to each exercise were compared by loading scheme design, using repeated measures analysis of variance. Tukey's post hoc analysis was performed to determine where the significant differences occurred. The criterion level for statistical significance was set at p ≤ 0.05.
It can be observed in Table 1 that single-repetition forces increased with load intensity (p < 0.001), although no significant differences were found between the 75 and 88% 1RM loads in terms of the eccentric forces developed during the Smith squat (p > 0.05). When comparing total-repetition data, the hypertrophy scheme produced greater forces than the other two schemes (p < 0.001), but no differences were found between the power and maximal strength schemes (p > 0.05).
No significant differences in repetition contraction times were observed for the 75 and 88% 1RM loads (p > 0.05) (Table 2). However, contraction times for the 45% 1RM mass were shorter than the two heavier intensities (p < 0.001). Despite the repetition similarities between the 75 and 88% 1RM loads, total-repetition contraction times were longer in the hypertrophy scheme (p < 0.001). Furthermore, when comparing the power and maximal strength schemes, total-repetition contraction times were found to be longer in the lighter condition (p < 0.001).
A comparison of single repetitions generally revealed greater impulses when heavier masses were lifted (Table 3). The only exception was the eccentric impulse for the Smith squat, where no significant differences were found between the impulses for the 75 and 88% 1RM loads (p > 0.05). In terms of total-repetition impulses, the hypertrophy scheme again produced the greatest responses (p < 0.001). For the power and maximal strength schemes, greater total-repetition impulses were associated with the heavier condition (p < 0.001).
As observed in Table 4, an increase in load produced a significant increase in repetition work on the supine (eccentric phase) and Smith squat (concentric phase) exercises (p < 0.001). For the concentric phase of the supine squat, the 88% 1RM load produced greater work than either the 45 or 75% 1RM loads (p < 0.001), but no differences were found between the two lighter intensities (p > 0.05). For the eccentric phase of the Smith squat, the 75 and 88% 1RM loads resulted in similar repetition work, with these values greater than that for the 45% 1RM load (p < 0.001). Total-repetition work was again superior in the hypertrophy scheme, with the power scheme also producing greater total work than the maximal strength scheme (p < 0.001), except for the eccentric phase of the Smith squat (p > 0.05).
Single-repetition power differed considerably across intensities (Table 5). For the supine squat, the 45% 1RM load maximized concentric power, with the 88% 1RM response also greater than that to the 75% 1RM load (p < 0.001). During the eccentric phase of this exercise, a difference was found only between the two heavy loads, with greater repetition power associated with the 88% 1RM intensity (p < 0.05). For the Smith squat, concentric repetition power was similar between the 45 and 88% 1RM loads (p > 0.05), with both exceeding the power with the 75% 1RM load (p < 0.001). No differences in repetition eccentric power were found (p > 0.05). In terms of total-repetition power, the hypertrophy scheme produced the greatest responses, with the power scheme also superior to the maximal strength scheme (p < 0.001).
The results of the single-repetition analysis were consistent with other research (2,3,8,24-26), with an increase in load intensity leading to greater forces, contraction times, impulses, and work, whereas power production varied across loads. However, the total-repetition responses were all superior in the hypertrophy scheme, compared with the power and maximal strength schemes, because of the greater number of repetitions performed (volume) as well as differences in lifting technique. Comparisons between the equal-volume power and maximal strength schemes further highlight limitations with single-repetition analysis: despite differences in repetition kinematics and kinetics (88% 1RM > 45% 1RM), the total-repetition responses were mostly greater for the lighter scheme. The interaction of volume, technique, and load therefore plays an important role in determining the mechanical responses to these workouts and, thus, may better describe the workout stimulus. These findings provide a framework for understanding inconsistencies within research and the prescription of loads for inducing specific adaptations.
The development of high contractile forces is thought necessary for inducing muscle growth and increasing maximal strength (20,22). Maximal or near-maximal contractions are important in recruiting the larger high-threshold motor units, to learn more efficient patterns of neural control and to stimulate protein synthesis in the fibers comprising these units (22). In line with other research (7,8,14,25), single-repetition forces generally increased with load intensity, which provides the basis on which resistance exercise is often prescribed (i.e., heavy loads = high contractile forces). However, such an approach does not reflect the actual workout stimulus, which typically comprises of multiple lifts performed across several sets and exercises. The hypertrophy scheme produced greater total forces than the maximal strength scheme, mainly because of the greater number of repetitions performed (100 vs. 24, respectively)-hence the volume of load lifted across the different workouts. The similarities in total forces between the 45 and 88% 1RM schemes may also be attributed to the respective numbers of repetitions performed (48 vs. 24) to equate these conditions by volume. Cronin and Crewther (7) observed a similar finding when examining the kinematic and kinetic responses of three equal-volume conditions (30, 60, and 90% 1RM) using ballistic squats. Despite repetition force increasing as per the load effect, the number of lifts performed in each condition (6, 3 and 2, respectively) resulted in greater total forces produced in the lightest condition.
Lifting technique is similarly important as a determinant of the workout responses to the power and maximal strength schemes. For example, the 45% 1RM load was moved ballistically (supine squat) and explosively (Smith squat), whereas the 88% 1RM load was moved using a traditional method for both exercises. Ballistic movements allow a given load to be accelerated for a greater proportion of the concentric phase than do traditional lifting techniques, thereby having a potentiating effect on the amount of force generated and related variables (8,24,25). Such findings may explain those studies that have found light- and heavy-load training equally effective for inducing hypertrophy and maximal strength development (10,12,19,23). That is, light-load ballistic training may provide an effective stimulus for adaptation by developing contractile forces comparable with those in heavier-load training, if equated by the volume of load lifted.
In conjunction with high forces, the training stimulus needs to be of sufficient duration to facilitate strength improvements (6). The longer a muscle is subjected to a training stimulus (i.e., time under tension), the greater the potential for adaptation to occur. The slower velocities associated with heavy loads increase contraction time, if single repetitions are compared (7,8,25). Accordingly, the 75 and 88% 1RM loads were characterized by longer repetition contraction times, in comparison with the 45% 1RM load. Lifting technique is also important; this is confirmed by the similarities between repetitions for the two heavier intensities. That is, the controlled manner in which the 75% 1RM load was moved may account for the slower velocities associated with the 88% 1RM load. Keogh et al. (14) have emphasized the importance of lifting technique on time under tension using a bench press exercise. Comparing several methods commonly employed by various practitioners, time under tension across a single set ranged from 7 seconds for maximal power training, 21-30 seconds for heavy weight training, isokinetics, eccentrics, isometrics, rest pause, and breakdowns, and 62 seconds for a super slow motion method (14). Because of the volume of load lifted across each workout, as discussed, total contraction times were longer in the hypertrophy scheme. Therefore, the amount of time under tension would seem a function of volume, load, and technique.
In terms of the two equal-volume schemes, the power scheme again resulted in longer total contraction times than the maximal strength scheme. This is not a novel finding, having been reported elsewhere (7). If time under tension mediated those adaptive changes occurring with training, then these results may also explain some of the disparities cited previously. Although the three loading schemes were similar in workout duration, the shorter rest periods in the hypertrophy scheme, combined with the longer total contraction times, produced a much smaller ratio of tension time to recovery time (1:6) than the power (1:15) and maximal strength schemes (1:20). As a result, this type of training (hypertrophy) is likely to impose greater stress on the endocrine and metabolic systems, which would seem necessary for muscle growth to occur (16,17). The authors recognize that the recovery time between sets might also influence set and exercise kinematics and kinetics but, given the approach taken in this study (single- vs. total-repetition analysis), these interactions will not be addressed in this paper.
Given the relative importance of high forces and time under tension, it may be argued that the development of large impulses is the critical stimulus for strength adaptation. In developing greater forces and longer contraction times per repetition, it is not surprising that heavy loads also maximize repetition impulses (7), as we found. Still, this finding has little practical relevance for understanding the mechanical stimuli afforded by training workouts. The hypertrophy scheme produced the largest total impulses, with differences in load volume providing the most likely explanation. In contrast to our previous findings, total-repetition impulses were found to be greater in the maximal strength scheme than the responses afforded by the power scheme. Thus, it would seem that heavy loads maximize the integration of force and time compared with lighter conditions of equal volume (7). If impulse production were the most important stimulus for muscular adaptation, then heavy-load training would seem superior to lighter-load training, both on the basis of single repetitions and across sets and workouts of equal volume. Unfortunately, the assessment and discussion of impulse as a training stimulus is not well documented. Our understanding of weight training may benefit from research differentiating between impulse, as opposed to force and time under tension, and mapping adaptations thereafter.
It is possible that changes in muscle size and strength also relate to the distance over which force acts, or work (6). By definition, work implies the development of force superimposed with changes in muscle length, or a stretching component, to provide an additional stimulus for muscle growth (22). Hormone release (e.g., growth hormone, testosterone), which is important for hypertrophy, is also associated with the amount of work performed across a workout (5,16,17). As expected, work generally increased with load intensity on a single-repetition basis, but the accumulative responses were again superior in the high-volume hypertrophy scheme, a finding supported by other research (4,5,7). For instance, Brown and colleagues (4) examined three groups of males (weight trained, endurance trained, untrained), each performing three sets of exhaustive leg presses at 60, 70, and 80% 1RM. Despite repetition work increasing with mass, total work was superior in the lightest condition because of the number of repetitions performed to failure with the 60% 1RM (37-46 repetitions), 70% 1RM (14-22 repetitions), and 80% 1RM loads (6-10 repetitions). As we found, when equated by volume, greater total work is associated with lighter loading conditions (7). The power and strength scheme differences in total work may also be attributed to the greater distances traveled across the supine (ballistic) and Smith (up to the toes) squat exercises when lifting the 45% 1RM load. These data may further explain some of the disparate findings between heavy- and light-load training and subsequent adaptations.
In theory, training loads that maximize mechanical power also optimize the contribution of force and velocity, thereby potentially benefiting a wider range of performance measures than either light-load (high velocity) or heavy-load (high force) training (6). We observed considerable variability in repetition power, which is consistent with other studies examining single and/or multiple repetitions (2,3,5,7,8,14,25,26). This variation may be attributed to the mode of dynamometry, the exercise performed and/or contraction type, lifting technique, and subject training status. Nonetheless, total power followed a similar trend to previous results (hypertrophy > power > maximal strength) to confirm the effect of load volume on the workout response. In maximizing total-repetition power, the hypertrophy scheme may explain those studies that have found heavy-load training (> 70% 1RM) as effective as light-load training in enhancing various power measures (1,11,21). In fact, strength is the basic quality that influences mechanical power; thus, improvements in muscle size (and force potential) may also benefit power adaptation. Even so, the neuromuscular contribution to maximal power development involves several components, including the maximal rate of force development, muscular strength at slow and fast repetition velocities, stretch-shortening cycle performance, and movement pattern and skill coordination (18). Therefore, adopting a more complex analysis of training methods used to enhance power development (e.g., traditional heavy weight training, explosive light-load training, plyometrics, combined plyometrics and weight training, maximal power training) (6), as we have, may help to elucidate those mechanical stimuli underpinning these adaptive changes.
In conclusion, our results show that although load intensity is generally the major determinant of the mechanical responses to single repetitions, it is the interaction of volume, technique, and load that determines the overall mechanical responses to these workouts-hence, the workout stimulus. These findings highlight limitations with using information based on single-repetition analysis (i.e., loading guidelines) and also provide a framework for understanding disparities cited within the literature regarding the effectiveness of different loading strategies for hypertrophy, maximal strength, and power adaptation.
On the basis of the results of the present study, it seems that volume, technique, and load all play important roles in determining the mechanical responses to a resistance exercise workout. If these responses provided the most potent stimuli for adaptations to occur, then changes in muscle size, strength, or power might be achieved using different training strategies. For instance, light-load ballistic training might be employed to stimulate high-velocity-specific adaptations, especially if the training velocities approached the actual movement velocity of the athletic event. Adopting such a strategy may also contribute to the development, or at least maintenance, of muscle size and maximal strength if a high-volume of training were also performed. Thus, greater focus could be placed on developing the functional characteristics of muscle without unduly compromising any hypertrophic and strength gains. Alternatively, less time might be spent on developing muscle size and maximal strength using more traditional heavy-load training methods, if similar results could be achieved using a more sports-specific approach (i.e., lighter loads, greater velocities) to resistance training.
This project was supported by a grant from the Health Research Council of New Zealand and the Maori Education Trust.
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